When an engine inlet is accepting the full volume of airflow passing through the projection of its intake aperture, the engine inlet is said to be operating in "full" mode. If for some reason, such as the inability of the engine to pass the full airflow or other restriction, the engine inlet cannot accept the full volume of the approaching airstream, the excess air is spilled around the external cowl, and the aircraft engine inlet is said to be operating in "spill" mode.
In general, subsonic aircraft engine inlets operate efficiently in spill mode during all phases of flight. Only small amounts of drag are incurred if cowls with blunt leading edges and streamlined external surfaces are used. For known supersonic aircraft, engine inlets operate in full mode during supersonic cruise flight and spill mode during subsonic flight. Supersonic engine inlets, however, are optimized for cruise conditions, where relatively sharp forward cowl edges and slender external cowl surfaces effectively reduce drag during full mode operation. Unfortunately, this optimum supersonic inlet shape is nearly the opposite of the optimum subsonic shape. The supersonic inlet design spills air inefficiently in the sub- and transonic flight segments (i.e., those flight segments prior to and after the supersonic flight segments) and causes the aircraft to incur high spillage drag. The result of this engine inlet flow spillage is an increase in the overall aircraft aerodynamic drag, which ultimately reduces the net thrust of the propulsion system.
Flow spillage around the supersonic engine inlet could be avoided by using an internal compression type of supersonic inlet, provided such an inlet were equipped with a variable capture area. In spite of their aerodynamic appeal, these inlets have not been proposed for practical application because of the difficulty in recovering from an abnormal operating mode called "unstart" during supersonic flight. Unstarts are described below.
As background information, supersonic airplanes can move faster than the local speed of sound (Mach 1) relative to the ambient air. Gas turbine engines propelling such airplanes work efficiently only if the approaching airflow is moving at subsonic speeds (generally less than Mach 0.6). It is the function of the shape of the engine inlet to reduce the speed of the intake airstream from supersonic to midrange subsonic Mach numbers. Therefore, the inlet must first slow the intake air stream to a near sonic speed (near Mach 1), then slow the airstream again to the desired subsonic speed. A supersonic inlet therefore comprises two distinct regions: a supersonic compression region and a subsonic compression region.
Various types of engine inlets are available and are defined by the amount of exposure the supersonic compression region bears relative to the external environment. At least three types of engine inlets are currently available: internal compression, mixed compression, and external compression. The present invention is concerned only with the internal compression type engine inlet, where supersonic compression occurs entirely within the interior of the engine inlet duct.
It is a physical characteristic of a gas, such as air, that in order to compress a supersonic stream (i.e., to reduce its Mach number) the inlet duct cross-sectional area must contract, but to continue the compression from Mach 1 to a lower Mach number, the cross-sectional area must expand. Therefore, all internal compression supersonic inlets have a convergent-divergent duct cross-sectional area distribution. The convergent duct area, termed the supersonic diffuser, defines the supersonic compression region. The divergent duct area, termed the subsonic diffuser, defines the subsonic compression region.
A cross-sectional side view of a prior art internal compression inlet 20 is illustrated in FIG. 1. The inlet 20 of FIG. 1 consists of a circular cowl and duct. The information presented with regard to the prior art is also relevant with regard to prior art rectangular inlet ducts. The internal compression inlet 20 includes an engine cowl 22 having a cowl lip 24 at its leading edge, and a duct defined by interior surfaces of the engine cowl. The duct includes three general regions: a supersonic diffuser 28 having convergent opposing walls; a subsonic diffuser 30 having divergent opposing walls and lying aft of the supersonic diffuser 28; and a small throat region 32 connecting the two diffusers and having generally parallel opposing walls. The throat region 32 is the area of the duct which is most narrow.
Still referring to FIG. 1, the aircraft on which the inlet 20 is mounted, normally travels at supersonic speeds. Free stream air (represented by arrow 34) external to the inlet is traveling at supersonic speeds relative to the inlet 20. The free stream air is captured and initially slowed from a first supersonic speed to a second supersonic speed within the supersonic diffuser 28. Various oblique shock waves are generated, represented by broken lines 36. The oblique shock waves 36 represent the dramatic transition of supersonic airflow to a slower supersonic speed. The airflow transitions from supersonic to subsonic speed across a normal shock wave 38 located in the forward area of the subsonic diffuser 30, just aft of the most narrow portion of the throat 32. The captured air is further slowed to lower subsonic speeds in the subsonic diffuser 30.
It is physically impossible for any normal shock wave to persist in the converging supersonic diffuser 28 of an internal compression engine inlet duct. If a flow disturbance causes the normal shock to move forward into a converging cross section, ahead of the throat, the flow becomes unstable, and the normal shock wave will be violently expelled out the front of the inlet until it takes up a position forward of the engine inlet aperture, at the cowl lip. This expulsion of the normal shock is commonly termed an "unstart" and is accompanied by abrupt large amplitude changes in the aerodynamic forces acting on the inlet. Flow disturbances leading to inlet unstarts have a number of natural causes, such as thermal updrafts or jet streams in the atmosphere. They may also be induced by aircraft maneuvers, or by changes in the airflow demand of the engine.
When an internal compression inlet unstarts, the thrust produced by the engine is significantly decreased, and at the same time, the drag caused by the inlet is significantly increased. The simultaneous decrease in thrust and increase in drag have a drastic effect on the aircraft's flight characteristics. If the aircraft is traveling at very high speeds when unstart occurs, the aircraft will rapidly decelerate, making control of the aircraft by the pilot difficult or impossible for a period of time. Unless the inlet is quickly "restarted" to move the normal shock from at, or forward of, the cowl lip back into the throat, the pilot will not be able to continue supersonic flight.
Normal operation is restored through the "restart" sequence. At each flight Mach number there is a maximum ratio of the aperture area (A) to the throat area (A*), above which restart is not possible. This well-known starting contraction ratio (A/A*) is much less than what is required for efficient supersonic aircraft performance. Nonetheless, if an unstart occurs, the ratio must be decreased to at least the maximum starting contraction ratio. Restart of an internal compression inlet is difficult because the inlet aperture area includes all of the engine's intake capture area such that a very large increase in the throat area is necessary to reach the starting contraction ratio.
Attempts to solve the problem of unstarts and restarts for internal compression inlets have been unsuccessful. U.S. Pat. No. 4,991,795 describes a configuration that has addressed the problem for mixed compression inlets. The device of the '795 patent includes a cowl, a forward ramp, an aft ramp, multiple actuators for selective movement of the ramps, and a slot between the forward and aft ramps. The slot is positioned at the inlet throat and opens into a plenum having a vent through which air may be expelled. A control system receives input from pressure sensors at various locations in the inlet and causes the position of a vent door to open or close, thereby controlling the pressure within the duct in order to control the position of the normal shock wave. Although the technique disclosed in U.S. Pat. No. 4,991,795 is intended for use with a mixed compression inlet, the technique is incorporated herein by reference, to the extent consistent with this disclosure.
Thus a need exists for an internal compression engine inlet that avoids spill mode and its associated spillage drag, deters unstarts from occurring, and allows quick restarts during supersonic flight should an unstart occur.